DE 296 08 342.9 U1 discloses a vent valve of this type in which the valve membrane is held by a clamp disk with air passage openings in the valve housing which is designed to be used at the same time for protection of the valve membrane against penetration of liquid from the tank into the valve for surge vibrations which occur during transport. Practice shows that the clamp disk does not adequately perform this protective function, since during transport the liquid can enter the valve housing and due to the suction action exerted by the liquid the valve membrane opens and liquid emerges from the container via the vent valve. Another major disadvantage of the known vent valve is that the valve membrane does not have the pressure tightness prescribed for transport and storage containers for liquids at a container pressure of 1 bar.
DE 25 01 228 A1 describes a closing cap which is designed for liquid containers, which can be screwed onto a drum neck and into which a safety valve which acts as a ventilation and bleeder valve is integrated which responds to overpressure and underpressure in the container. In this safety valve a valve disk of elastic material which is highly prestressed during installation and thus assumes the shape of a spherical segment is used. In this valve there is the danger of loss of tightness as a result of fatigue phenomena of the valve disk material during prolonged use.